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Thickness-Controlled Synthesis of CoX2 (X = S, Se, and Te) Single Crystalline 2D Layers with Linear Magnetoresistance and High Conductivity

Xingguo Wang, Zhang Zhou, Peng Zhang, Shuqing Zhang, Yang Ma, Weiwei Yang, Hao Wang, Bixuan Li, Lingjia Meng, Huaning Jiang, Shiqiang Cui, Pengbo Zhai, Jing Xiao, Wei Liu, Xiaolong Zou, Lihong Bao,* and Yongji Gong*

Cite This: Chem. Mater. 2020, 32, 2321−2329

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ABSTRACT: Two-dimensional (2D) materials especially transition metal dichalcogenides (TMDs) have drawn intensive interest owing to their plentiful properties. Some TMDs with magnetic elements (Fe, Co, Ni, etc.) are reported to be magnetic theoretically and experimentally, which undoubtedly provide a promising platform to design functional devices and study physical mechanisms. Nevertheless, plenty of theoretical TMDs remain unrealized experimentally. In addition, the governable synthesis of these kinds of TMDs with desired thickness and high crystallinity poses a tricky challenge. Here, we report a controlled preparation of CoX2 (X = S, Se, and Te) nanosheets through chemical vapor deposition. The thickness, lateral scale, and shape of the crystals show great dependence on temperature, and the thickness can be controlled from a monolayer to tens of nanometers. Magneto-transport characterization and density function theory simulation indicate that CoSe2 and CoTe2 are metallic. In addition, unsaturated and linear magnetoresistance have been observed even up to 9 T. The conductivity of CoSe2 and CoTe2 can reach 5 × 106 and 1.8 × 106 S/m, respectively, which is pretty high and even comparable with silver. These cobalt-based TMDs show great potential to work as 2D conductors and also provide a promising platform for investigating their magnetic properties.

predicted by theoretical calculation, till date have hardly been explored on the scale of atomic layers experimentally,34 and the scalable preparation of these TMDs with controllable layer numbers and high quality still remains a serious challenge. For instance, layered TMD CoX2 (X = S, Se, and Te) is composed of hexagonal unit cells, where each Co atom is bonded with six X (chalcogen) atoms. Because of the existence of cobalt elements, CoX2 is possible to behave as a magnet with atomic thickness. A few studies have been conducted on the preparation and application of CoX2. For example, a nonlayered material CoS2 is reported as ferromagnetic metal with the Curie temperature TC of 124 K, and CoSe2 with the pyrite-type structure is regarded as the controversial Pauli paramagnet or the antiferromagnet.35−37 Some initial efforts have also been made on CoTe2 in two dimensions recently. Duan and co-workers reported the layer-controlled growth of

1. INTRODUCTION

The family of two-dimensional (2D) materials such as graphene,1,2 h-BN,3 black phosphorus,4,5 CrI3,6 FePS3,7 and transition metal dichalcogenides (TMDs) has shown exciting potentials in various aspects, including both scientific research (superconductivity, charge density wave, magnetism, topological properties, etc.)8−14 and practical applications (electronics, optoelectronics, catalysis, energy storage, etc.).15−25 Among these, TMDs have drawn intensive attention because of their abundant types and thickness-dependent properties.26,27 For example, the superconducting transition temperature of NbSe2 nanosheets is strongly dependent on their layer numbers.28 Three different charge−density−wave phases have

been observed in TaS2.29 Linear magnetoresistance (MR) and topological superconductivity have also been discovered in WTe2 nanosheets.30 Recently, some TMDs with magnetic elements such as CrSe2, CrTe2, VTe2, VTe2, and so forth have been predicted theoretically or proved experimentally to have some magnetic properties,31−33 which can be served as a powerful platform to design functional devices and study physical magnetic mechanisms. Generally, plenty of fundamental physical phenomena and potential applications can be realized in 2D TMDs. However, some new kinds of TMDs,

Received: October 28, 2019 Revised: March 4, 2020 Published: March 4, 2020

https://dx.doi.org/10.1021/acs.chemmater.9b04416

© 2020 American Chemical Society

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Figure 1. Preparation and morphology of CoX2 (X = S, Se, and Te). (a) Schematic illustration of the growth process. Mixture of Co3O4 and NaCl are used as Co precursors which can react with S or Se or Te gas at a certain temperature and atmosphere. (b−d) OM images and height profiles of

ultrathin CoS2, CoSe2, and CoTe2 nanoplates on SiO2/Si. (e) OM image of the large view of CoTe2 with uniform thickness. (f) Diagrams of the smallest thickness of CoTe2 nanosheets plotted as a function of the growth temperature. (g) Evolution of the largest lateral scale of CoTe2 nanosheets with the growth temperature.

CoTe2 and CoSe and studied the electric properties of the resulting nanosheets.38,39 Wang and co-workers found tunable magnetic properties of CoTe2 treated with different concentrations of NaOH.40 However, high-quality ultrathin-layered CoX2 (X = S and Se) has not been synthesized yet. Moreover, CoX2 nanosheets with well-controlled thickness down to the monolayer are desirable to explore the layer-dependent magnetic properties of these materials. the conductivity of silver showing promising application potential as 2D conductors. According to the magnetotransport and superconducting quantum interference device (SQUID) test, both CoSe2 and CoTe2 seem likely to be paramagnetic in nature. Density function theory (DFT) is further used to simulate the band structures of CoX2, further confirming their metallic nature. Altogether, the studies illustrate a controllable way to synthesize 2D CoX2, which may provide a promising opportunity for studying the properties of 2D CoX2 with various layers, such as electronic, magnetic, catalytic, and other properties.
Herein, we report a chemical vapor deposition (CVD) approach to synthesize few-layer CoX2 nanosheets with the controlled thickness on SiO2/Si substrates by tuning the growth temperatures. By using monolayer MoSe2 as substrates (monolayer MoSe2 developed on SiO2/Si substrates in advance), a single-layer CoX2 can be synthesized for the first time. In addition, the growth temperature plays an important role in the evolution of the lateral size and shape. Optical microscopy (OM) images illustrate the morphologies of the asgrown CoX2 nanosheets, mostly displaying a hexagonal or triangular shape with the lateral domain size ranging from ∼5 to ∼100 μm. Atomic force microscopy (AFM) studies reveal the thickness of the nanosheets varying from 1.18 to ∼20 nm. X-ray photoemission spectroscopy (XPS) is carried out to analyze chemical states, and Auger electron spectroscopy (AES) mapping indicates the compositional homogeneity of the as-grown CoX2 samples. The crystal structure and high crystalline quality are confirmed by high-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED) characterizations. Magneto-transport studies show that CoX2 (X = Se and Te) single crystals exhibit metallic behaviors. Additionally, unsaturated linear MR (LMR) up to 9 T occurs in both CoSe2 and CoTe2 nanosheets. Meanwhile, both CoSe2 and CoTe2 with various thicknesses show outstanding conductivity up to 106 S/m, which is close to

2. RESULTS AND DISCUSSION

The CoX2 (X = S, Se, and Te) nanosheets on 285 nm SiO2/Si substrates are synthesized using a traditional CVD system, as shown in Figure 1a, with the mixture of Co3O4 (0.1 g) and NaCl (0.03 g) as the Co source while different chalcogen element powders as X precursors. Although the growth time and the flow of carrier gas can also cause a certain influence on the thickness, which was reported in other TMDs such as NiTe2,41 the thickness of MX2 can still be tuned effectively by controlling the growth temperature in this study. More details of the sample synthesis are described in the Methods section. The OM images, as shown in Figure 1b−d, indicate that CoX2

(X = S, Se, and Te) nanosheets exhibit hexagonal or triangular shapes with various lateral size. The AFM results demonstrate that the typical thickness of CoS2, CoSe2, and CoTe2 is about 7.2, 5.1, and 3.9 nm, respectively. Figure 1e shows the OM image of the uniform CoTe2 nanosheets in a large view. In order to achieve controllable synthesis of these 2D materials, we take CoTe2 as an example and conduct systematic studies to investigate the effect of growth temperature on the thickness and lateral size. With the Ar/H2 flow

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Figure 2. Characterizations of chemical states and compositional homogeneity. (a and e) XPS spectra of Co 2p from CoSe2 and CoTe2 nanosheets. (b and f) XPS spectra of Se 3d and Te 3d. Indexed peaks are formed by CoSe2 or CoTe2, while the others come from some contaminations during the CVD process. (c,d,g, and h) AES mapping spectra of CoSe2 and CoTe2 nanosheets.

rate constant at 50 sccm in which 10% of the gas is H2 and the time of deposition process constant is 3 min, the nanosheets produced at different temperatures from 720 to 800 °C show a clear thickness evolution, as shown in Figure 1f. With increasing growth temperatures, thicker CoTe2 nanosheets are grown. However, once the temperature is lower than 740 °C, the lateral size of the nanosheets becomes smaller while the thickness remains almost unchanged. In addition, the revolution of the lateral size of CoTe2 with the temperature ranging from 720 to 800 °C is also studied when the deposition time and flow rate remain constant. It is obvious that the largest lateral size increases from several to tens of micrometers with the temperature increasing from 720 to 780 °C (Figure S2). When the temperature is further raised to 800 °C, the largest lateral size slightly fluctuates (Figure 1g). These data reflect that the material preferably grows in both vertical and lateral directions with the increasing temperature. Thermodynamics and kinetics are the two key factors to control the growth of crystals, which can be used to explain the aforementioned phenomena. As explained in the previous paper, vertical growth of 2D materials is resulted from the thermodynamic product because of the van der Waals force between adjacent layers, and in-plane growth of 2D materials is caused by the kinetic product because of the lower nucleation barrier when putting atoms to the edges with unsaturated bonds.42−44 It is well known that the thermodynamic product is preferred at high temperature and the kinetic product is preferred at low temperature. In our case, when the temperature is below 740 °C, the reaction is controlled by kinetics. In this situation, when increasing the temperature, the lateral size of the sample becomes larger because of more vapor from the precursors. When the temperature is higher than 780 °C, the reaction is dominated by thermodynamics. Thus, a higher temperature will result in a thicker sample rather than a larger size. With the temperature ranging from 740 to 780 °C, the competition between thermodynamics and kinetics leads to the increased thickness and size when increasing the temperature. to 780 °C. This phenomenon can be explained by the local changes in the Co/Te ratio at different temperatures, which has been discussed in the CVD growth of MoS2.45 Although the growth temperature and deposition time can effectively tune the thickness of CoX2, it still remains a challenge to obtain a single-layer CoX2 on the SiO2/Si substrate. However, as reported in the literature, obtaining single-layer samples is quite significant because the optical, electrical, and magnetic properties of 2D materials exhibit strong thickness dependence.6,46 We speculate that a singlelayer CoX2 may be unstable on the SiO2/Si substrate because of the high interface energy. Hence, when MoSe2/SiO2/Si is designed as the substrate, it turns out to obtain single-layer samples. The OM image of thin CoSe2 deposited on MoSe2/ SiO2/Si nanosheets is shown in Figure S4e, where different color contrasts indicate different phases. The AFM height profile image in Figure S4f shows that the thinnest CoSe2 nanosheets produced on MoSe2/SiO2/Si is 1.18 nm, indicating a single layer which probably results from smaller nucleation and growth energy on MoSe2/SiO2/Si. Consequently, the single-layer nanosheets of the other CoX2 (X = S and Se) may be obtained in the same way. Totally, the suitable growth temperature for CoTe2 may occur between 740 and 760 °C, during which a relatively thin (∼4 nm) and an averagely large lateral domain (∼20 μm) CoTe2 nanosheets can be obtained. CoS2 and CoSe2 nanosheets with different layer numbers can be synthesized by a similar route. Notably, CoX2 crystals can degenerate badly in air after some days while the oxidation of the samples happens slowly in an evacuated bag, as shown in Figure S5. Thus, in order to avoid possible oxidation of the samples, all the characterizations are conducted as soon as possible. Raman studies together with OM images of the as-grown ultrathin nanoplates are shown in Figures S7 and S1. The chemical states and the compositional homogeneity of the resulting CoX2 (X = Se and Te) samples were analyzed by XPS and AES mapping, respectively. The survey-scanning XPS spectra for CoSe2 and CoTe2, as shown in Figure S8, confirm the presence of Co, Se, and Te. Figure 2a,e demonstrates the Co 2p core-level spectra of CoSe2 and CoTe2. Two peaks at 780.0 and 796.8 eV correspond to Co2+ 2p3/2 and Co2+ 2p1/2 of CoSe2, and the peaks located at 778.4 and 793.7 eV may result
Meanwhile, there is morphological transition when the growth temperature changes (Figure S3). It is clear to see that the ratio of triangular nanosheets decreases while hexagonal crystals dominate when the temperature is increased from 740

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    UC Riverside UC Riverside Electronic Theses and Dissertations Title Two - Dimensional van der Waals Materials: Characterization and Electronic Device Applications Permalink https://escholarship.org/uc/item/4453831z Author Samnakay, Rameez Rauf Publication Date 2016 Peer reviewed|Thesis/dissertation eScholarship.org Powered by the California Digital Library University of California UNIVERSITY OF CALIFORNIA RIVERSIDE Two - Dimensional van der Waals Materials: Characterization and Electronic Device Applications A Dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Materials Science and Engineering by Rameez Rauf Samnakay August 2016 Dissertation Committee: Dr. Alexander A. Balandin, Chairperson Dr. Roger K. Lake Dr. Alexander Khitun Copyright by Rameez Rauf Samnakay 2016 The Dissertation of Rameez Rauf Samnakay is approved: Committee Chairperson University of California, Riverside Acknowledgements I would like to express my thanks and gratitude to a multitude of people who I have met and interacted with over the course of my life and who helped mould me and make what I am today. First, I would like to thank my advisor, Dr. Alexander Balandin, for giving me guidance and direction as well as giving me an opportunity to conduct research at the highest level. What I learned from his expertise and professionalism to approaching problems and research will remain me with me throughout my life. I would like to thank Dr. Roger Lake and Dr. Alexander Khitun for serving in my dissertation committee as well as providing invaluable advice during the course of my studies. In addition, I was lucky to interact and collaborate with some great researchers and scientists especially Dr.
  • Polytypism, Polymorphism, and Superconductivity in Tase2−Xtex

    Polytypism, Polymorphism, and Superconductivity in Tase2−Xtex

    Polytypism, polymorphism, and superconductivity PNAS PLUS in TaSe2−xTex Huixia Luoa,1, Weiwei Xiea, Jing Taob, Hiroyuki Inouec, András Gyenisc, Jason W. Krizana, Ali Yazdanic, Yimei Zhub, and Robert Joseph Cavaa,1 aDepartment of Chemistry and cJoseph Henry Laboratories and Department of Physics, Princeton University, Princeton, NJ 08544; and bDepartment of Condensed Matter Physics and Materials Science, Brookhaven National Laboratory, Upton, NY 11973 Contributed by Robert Joseph Cava, February 9, 2015 (sent for review January 14, 2015; reviewed by J. Paul Attfield and Maw-Kuen Wu) Polymorphism in materials often leads to significantly different 3R form (20–22). The 3R form can be synthesized, but it is not the physical properties—the rutile and anatase polymorphs of TiO2 are stable variant (the 2H form is) and so has been the subject of little a prime example. Polytypism is a special type of polymorphism, study. In one of the other polymorphs, the 1T (T: trigonal) type, occurring in layered materials when the geometry of a repeating Ta is found in octahedral coordination in the Se-Ta-Se layers and structural layer is maintained but the layer-stacking sequence of the layer stacking along the c axis of the trigonal cell such that the the overall crystal structure can be varied; SiC is an example of structure repeats after only one layer (23) (Fig. 1A). Again, the 1T a material with many polytypes. Although polymorphs can have form has not been the subject of much study. Here we show that radically different physical properties, it is much rarer for polytyp- the 3R and 1T polymorphs are both quite stable in the TaSe2−xTex ism to impact physical properties in a dramatic fashion.